Champion visual overlays are all visible at once while spectating games.

This is easiest to see if you spectate any game with Evelynn, since her passive post-6 tints the screen while she's stealthed.

The more champions there are in the game with overlays, the more distracting it tends to get in teamfights. Like when you have a stealthed Eve looking for a target, while PROJECT Vayne has ulted and Urgot just triggered his execute on someone.